Classical glass-ceramics had their genesis in U.S. Pat. No. 2,920,971. As explained there, the production of glass-ceramics contemplates the controlled in situ crystallization of precursor glass bodies and involves three general steps: (1) a glass forming batch customarily containing a nucleating agent is melted; (2) that melt is cooled and simultaneously a glass body of a desired configuration is shaped therefrom; and (3) that glass body is subjected to a predetermined heat treatment which initially develops nuclei in the glass upon which crystals subsequently grow. Because glass-ceramic articles are typically highly crystalline, i.e., greater than about 50% by volume crystalline, the properties exhibited thereby are more closely akin to those of the crystal phase than to those of the parent glass. Furthermore, because classical glass-ceramic articles are prepared through the controlled in situ crystallization of glass articles, they are non-porous and free of voids.
The hallmark of classical glass-ceramic articles consists of the presence of a nucleating agent, e.g., the TiO.sub.2 disclosed in U.S. Pat. No. 2,920,971. Whereas the inclusion of a nucleating agent normally insures a high degree of crystallization uniformly distributed throughout the final product, its presence may also contribute undesirable effects. For example, TiO.sub.2 is a high temperature flux which can lower the refractoriness of the glass-ceramic. Also, crystals of the nucleating agent, rutile for instance, can form in the glass-ceramic, thereby modifying the properties demonstrated by the glass-ceramic. Finally, the presence of a nucleating agent can lead to the growth of an unwanted crystal phase.
In view of those circumstances, glass-ceramic articles have been produced by sintering glass powders containing no nucleating agent as such into an integral body; instead relying upon surface nucleation from the very finely-divided grains of glass. In general, the glass powders will pass a No. 325 United States Standard Sieve (44 microns) and, most preferably, will have a size not exceeding 10 microns. This production practice permits, with the careful control of glass composition, the fabrication of glass-ceramic articles containing a particularly desired crystallization.
The process for producing glass-ceramic articles by sintering glass powders comprises four general steps: (1) a glass forming batch is melted; (2) that melt is cooled and the resulting glass is comminuted to a very finely-divided powder (commonly termed "frit"); (3) that powder is formed into a desired shape; and (4) that shape is fired at a sufficiently high temperature to sinter the glass particles together into an integral body and to cause the growth of crystals therewithin.
Unfortunately, this last consolidation step has customarily required the use of very high temperatures to produce, without microcracking, essentially void-free and non-porous bodies; i.e., bodies exhibiting substantially theoretical density. For example, .beta.-spodumene (keatite solid solutions) materials contain large amounts of Li.sub.2 O and are difficult to sinter to high density, as are zinc .beta.-quartz compositions which demonstrate high expansion hysteresis and must be sintered in the neighborhood of or in excess of 1000.degree. C. Cordierite glass powders are likewise difficult to sinter to near theoretical density unless a second crystal or a glass phase is also present.